As background to a discussion of stents, one notes that in the 1970s, the technique of percutaneous transluminal coronary angioplasty (PTCA) was developed for the treatment of atherosclerosis. Atherosclerosis is the build-up of fatty deposits or plaque on the inner walls of a patient's arteries; these lesions decrease the effective size of the artery lumen and limit blood flow through the artery, prospectively causing a myocardial infarction or heart attack if the lesions occur in coronary arteries that supply oxygenated blood to the heart muscles. In the angioplasty procedure, a guide wire is inserted into the femoral artery and is passed through the aorta into the diseased coronary artery. A catheter having a balloon attached to its distal end is advanced along the guide wire to a point where the sclerotic lesions limit blood flow through the coronary artery. The balloon is then inflated, compressing the lesions radially outward against the wall of the artery and substantially increasing the size of its internal lumen, to improve blood circulation through the artery.
Presently, it is the case that stents are increasingly being used in place of or in addition to PTCA for treatment of atherosclerosis, with the intent of minimizing the need to repeatedly open an atherosclerotic artery. Although a number of different designs for stents have been published, stents are generally configured as elongate cylindrical structures that are provided in a first state and can assume a second, different state, with the second state having a substantially greater diameter than the first state. A stent is implanted in a patient using an appropriate delivery system for the type of stent being implaced within the patient's arterial system. There are two basic types of stents—those that are expanded radially outward due to the force from an inflated angioplasty type balloon, such as the Bx Velocity® and Palmaz-Schatz® stents, made by Cordis Corporation, and those that are self expanding, such as the SMART® stent, made by Cordis Corporation.
Generally, stents, grafts, and graft stents are implantable medical devices (sometimes termed implantable tubular prostheses) which are placed within blood vessels and other body passageways to treat disease conditions such as stenoses, occlusions, and aneurysms. That is, a stent is used as a tubular structure left inside the lumen of a duct to relieve an obstruction. Commonly, stents are inserted into the lumen in a non-expanded form and are then expanded autonomously (or with the aid of a second device in situ. A typical method of expansion occurs through the use of a catheter mounted angioplasty balloon which is inflated within the stenosed vessel or body passageway in order to shear and disrupt the obstructions a associated with the wall components of the vessel and to obtain an enlarged lumen. Transluminal implantation of such devices requires that they be introduced to the site collapsed about or within an introduction device and released to self expand or are expanded by other mechanisms to an expanded tubular state providing a lumen of approximately the same size as the patent vessel or duct lumen.
In the absence of a stent, restenosis may occur as a result of elastic recoil of the stenotic lesion. A number of stent designs have been reported. Such stents include those with rigid ends (8 mm) and a flexible median part of 7–21 mm. This device is formed of multiple parts and is not continuously flexible along the longitudinal axis. Other stent designs with rigid segments and flexible segments have also been described.
Other stents are described as longitudinally flexible but consist of a plurality of cylindrical elements connected by flexible members. These designs have at least one disadvantage if, for example, protruding edges occur when the stent is flexed around a curve, raising the possibility of inadvertent retention of the stent on plaque deposited on arterial walls. This may cause the stent to cause some damage to the interior lining of healthy vessels.
Stents can be viewed as scaffoldings, of generally cylindrical symmetry, that function to physically support, and, if desired, expand the wall of the passageway. Typically, a stent consists of two or more struts or wire support members connected together into a lattice-like or open weave frame. Most stents are compressible for insertion through small cavities, and are delivered to the desired implantation site percutaneously via a catheter or similar transluminal device. Once at the treatment site, the compressed stent is expanded to fit within or expand the lumen of the passageway. Stents are typically either self-expanding or are expanded by inflating a balloon that is positioned inside the compressed stent at the end of the catheter. Intravascular stents are often deployed after coronary angioplasty procedures to reduce complications, such as the collapse of arterial lining, associated with the procedure.
Stents have a lattice-like structure, which leaves spaces defined by the struts that form the stent. Such spaces can allow plaque from the lesion to fall through the stent and enter the blood stream during stent deployment. The spaces can also permit malignant tissue growth through the stent openings into the body passageway and can allow undesired contact between blood flowing through the blood vessel and damaged portions of the vessel. Covered stents, in which a polymeric material surrounds and is attached to the stent, have been proposed to alleviate the problems associated with stent openings.
Diseased vessels are also treated with grafts. Grafts are generally tubular in morphology and are used to replace or create an anatomical passageway to provide a new conduit for fluid, e.g. blood. Grafts are often made from a portion of a vein, but can also be constructed from a synthetic material to form a synthetic graft. Like stents, synthetic grafts can be positioned percutaneously via a catheter, for instance, to be placed at the site of an aneurysm to prevent further dilation and possible rupture of the diseased vessel. In certain instances, the graft material alone does not provide enough structural support for the graft, causing the graft to at least partially collapse and occlude or impede the flow of blood through the vessel. Grafts may be used with stents. For those cases wherein the graft material is synthetic, the combined structure is sometimes referred to as a synthetic stent-graft. Stents are also placed at the ends of synthetic grafts to help secure the ends of the synthetic graft to vessel walls.
The present invention pertains to a manner of arranging the flexible connectors of a stent to reduce the friction between the stent and the wall of the vessel during delivery.
The present invention also reduces the likelihood of protruding edges that occur when the stent is flexed around a curve which increase to a certain degree the possibility of retention of the stent on plaque deposited on arterial walls.